WHAT IS A GENE? 153 



POSITION PSEUDOALLELISM 



Evidence for this phenomenon has been found repeatedly in higher 

 forms, especially in Drosophila, where it was first detected and exten- 

 sively studied bv E. B. Lewis. In this fly, for example, there is a region 

 in the first chromosome, about 0.2 of a map unit in length, in which 

 are located at least three genetic units called lozenge ' h ■. They have, 

 in mutant form, similar eflFects on the morpholog)- and pigmentation of 

 the eves. Each factor is recessive when present singly, e.g.,: 



^i — — — — (normal! 



But in heteroz\gotes with t\vo diflFerent mutant factors the effect depends 

 upon w hether the normal units are on the same chromosome ' cis-arrange- 



ment ' e.g., 



h I- ~ 



^li ^ ( normal ' 



or whether one normal factor is on one chromosome and the other on its 

 homologue (fran^-arrangement . e.g.. 



^^i — j^ ^ (mutant* 



In the former case the phenot^•pe is normal; in the latter case it is 

 mutant. Thus, for normal hinction there must be one set of normal 

 factors on at least one chromosome. There is not only a position effect 

 but it is such that a fran.s-heteroz\gote for two independently mutable 

 and recombinable units shows a mutant character. We may ask w hether 

 it is possible in this case to be sure that the genetic subunits in this 

 complex form one large functional gene which can operate only when 

 all of its parts are normal. This question is especially difficult to ans>ver 

 in an organism such as Drosophila in which the biochemistr)- inter\ening 

 bet^veen the gene and a character such as the eye is not well understood. 

 .\ltematively, each lz~ unit mav be a separate gene and perform a dif- 

 ferent primar\' function, and these functions mav be linked in space and 

 time as follows: 



Biochemical Sequence 

 A ^ B ^ C 



\l 



_■;.+ ^,+ ;;.+ 



First chromosome of DrosophiJa 



